The use of recombinant host cells in the expression of heterologous proteins has in recent years greatly simplified the production of large quantities of commercially valuable proteins, which otherwise are obtainable only by purification from their native sources. Currently, there is a varied selection of expression systems from which to choose for the production of any given protein, including prokaryotic and eukaryotic hosts. The selection of an appropriate expression system will often depend not only on the ability of the host cell to produce adequate yields of the protein in an active state, but also to a large extent may be governed by the intended end use of the protein.
Although mammalian and yeast cells have been the most commonly used eukaryotic hosts, filamentous fungi have now begun to be recognized as very useful as host cells for recombinant protein production. Among the filamentous fungi which are currently used or proposed for use in such processes are Neurospora crassa, Acremonium chrysogenum, Tolypocladium geodes, Mucor circinelloides and Trichoderma reesei. In addition, certain species of the genus Aspergillus have been used effectively as host cells for recombinant protein production. Aspergillus is a deuteromycete fungus characterized by an aspergillum consisting of a conidiospore stipe terminating in a vesicle, which in turn bears one or two layers of synchronously formed specialized cells, variously referred to as sterigmata or phialides, and asexually formed spores referred to as conidia. The species Aspergillus nidulans has been reported to be transformed with recombinant plasmids (Ballance, et al. Biochem. Biophys. Res. Comm. 112: 284-289, 1983), but transformation was found to occur at fairly low frequency. Both Aspergillus niger and Aspergillus oryzae have also been described as being useful in recombinant production of proteins. However, other species of Aspergillus have not been shown to be useful in expression of heterologous protein, and in fact, because of poor expression and/or excessive production of proteases or mycotoxins, not all species of Aspergillus are suitable as host cells for this purpose, nor is this ability predictable from one species to the next. An ideal expression system is one which is substantially free of protease and mycotoxin production and large amounts of other endogenouslymade secreted proteins, and which is capable of higher levels of expression than known host cells. The present invention now provides new Aspergillus expression systems which fulfill these requirements.